BART on Friday released a statement explaining its actions after cellphone reception in four downtown San Francisco stations was interrupted Thursday in a move to disrupt a planned demonstration, a decision denounced by civil libertarians.

In the statement, BART officials said the transit agency cut cellphone service after learning protesters planned to use mobile devices to coordinate a demonstration over the July 3 police shooting of Charles Blair Hill on the Civic Center station platform. Hill, a transient, was shot after police say he threw a bottle at them and then approached a BART officer and displayed a weapon.

“A civil disturbance during commute times at busy downtown San Francisco stations could lead to platform overcrowding and unsafe conditions for BART customers, employees and demonstrators,” BART said in a statement on its website. “BART temporarily interrupted (cellphone) service at select BART stations as one of many tactics to ensure the safety of everyone on the platform.”

The statement also noted that it is illegal to demonstrate on the platform or aboard the trains.

BART said it has set aside areas for demonstrations, and that the agency manned its stations and trains with additional personnel in case of emergencies.

Still, the American Civil Liberties Union said shutting down cellphone usage was objectionable and expressed concern about passenger safety.

“Shutting down access to mobile phones is the wrong response to political protests,” the ACLU’s Rebecca Farmer said in a blog post.

No demonstrations developed Thursday. A July 11 demonstration disrupted service during the rush-hour commute, prompting the closure of the Civic Center and ending in many arrests.

BART police Chief Kenton Rainey said last month that he is “comfortable” with how officers reacted on the night Hill was killed.

The Associated Press contributed to this story. Contact Rick Hurd at 925-945-4780.